FIG. 13 is a block diagram of an electric vehicle power supply apparatus which is disclosed in patent literature 1. In an electric vehicle power supply apparatus 1 shown in FIG. 13, in the event that the load of an motor (M) 2 is small and that a driving voltage required by the motor (M) 2 is small, a first switch (SW1) 14 is off (open) and a second switch (SW2) 15 is on (closed), so that a first battery 11 and a second battery 12 are connected parallel to each other with respect to a driving inverter 3 for the motor (M) 2. On the other hand, in the event that the load of the motor (M) 2 is great and that a driving voltage required by the motor (M) 2 is great, the first switch (SW1) 14 is on and the second switch (SW2) 15 is off, so that the first battery 11 and the second battery 12 are connected in series with each other with respect to the driving inverter 3 for the motor (M) 2. In this way, in the event that the load of the motor (M) 2 is great, the driving voltage of the motor (M) 2 is increased so that a desired motor performance can be ensured, whereas in the event that the load of the motor (M) 2 is small, the driving voltage of the motor (M) 2 is prevented from being excessive so that the operation efficiencies of the motor (M) 2 and the driving inverter 3 can be increased.
In addition, in the electric vehicle power supply apparatus 1, in the event that the connection of the first battery 11 and the second battery 12 is switched from the parallel to the series in response to an increase in load of the motor (M) 2, the first switch (SW1) 14 and the second switch (SW2) 15 are off, so that the second battery 12 is cut off from the motor (M) 2, whereby electric power is supplied to the motor (M) 2 only by the first battery 11. Then, a DC-DC converter 13 continues to perform a boosting or voltage increasing operation until an electric potential VB at a second node B which is connected to the first switch (SW1) 14 becomes equal to the electric potential VC of a third node C which is connected to the first switch (SW1) 14, whereafter the DC-DC converter 13 stops performing the voltage increasing operation and the first switch (SW1) 14 is closed to be on. In this way, with the electric power supply to the motor (M) 2 maintained, the connection of the first battery 11 and the second battery 12 with respect to the motor (M) 2 is switched between the parallel and the series in response to the increase in load of the motor (M)2. By activating the DC-DC converter 13 to operate only when the connection of the first battery 11 and the second battery 12 is switched, compared with a case where the DC-DC converter 13 is kept operating at all times, it is possible to restrict an increase in switching loss in the DC-DC converter 13.